(EUV Lithography)
There is a trend in recent years to provide finer structures on semiconductor devices. With this trend, there has been proposed EUV lithography in which EUV having a wavelength of approximately 13.5 nm is used as a light source. EUV lithography, in which the light-source wavelength is short and light absorbency is very high, has to be conducted in a vacuum. In the wavelength range of EUV, most substances have a refractive index slightly smaller than 1. Therefore, EUV lithography cannot use transmission-type refractive optical systems that have been conventionally used, but has to use reflective optical systems. Therefore, in EUV lithography, conventional transmission-type masks cannot be used as a photomask (hereinafter referred to as the “mask”) that is an original plate, but reflective-type masks have to be used.
(Structures of EUV Mask and Blank)
A reflective mask blank, which is an original mask of such a reflective-type mask, includes a multi-layer reflective layer and an absorbing layer formed in this order on a low thermal expansion substrate. The multi-layer reflective layer has a high reflectance relative to the wavelength of an exposure light source. The absorbing layer absorbs the wavelength of the exposure light source. The substrate has a rear face on which a rear-face conductive film is formed for an electrostatic chuck in an exposure device. There is also an EUV mask having a structure in which a buffer layer is provided between a multi-layer reflective layer and an absorbing layer. In processing a reflective mask blank into a reflective mask, the absorbing layer is partially removed by electron beam (EB) lithography and etching. In the case of the structure having a buffer layer, the absorbing layer is similarly removed to form a circuit pattern composed of absorbing portions and reflecting portions. An optical image reflected by the reflective mask thus prepared is transferred onto a semiconductor substrate by way of a reflective optical system.
(Thickness of Absorbing Layer of EUV Mask and Reflectance)
In exposure methods using a reflective optical system, light is applied to a mask surface at an incident angle which is inclined by a predetermined angle (usually 6°) relative to a vertical direction. Accordingly, in the case where the thickness of the absorbing layer is large, the incident light casts a shadow of the pattern on the semiconductor substrate. Since the shadowed portions will have reflection intensity smaller than in the unshadowed portions, contrast is lowered in the transferred pattern, causing blurred edges or displacement from designed dimensions. This is called shadowing, which is one of the problems inherent to reflective masks.
In order to prevent blur in the pattern edges or displacement from designed dimensions, an effective way is to reduce the thickness of the absorbing layer and the height of the pattern. However, a reduced thickness of the absorbing layer degrades the light shielding properties of the absorbing layer, and also degrades transfer contrast and accuracy in the transferred pattern. In other words, when the absorbing layer is too thin, the contrast necessary to keep the accuracy in the transferred pattern will no longer be obtained. In other words, an absorbing layer, which is excessively thick or thin, can cause problems. Therefore, the thickness of the absorbing layer recently is in a range of about 50 to 90 nm, with the reflectance of extreme ultraviolet rays (EUV rays) of the absorbing layer being in a range of about 0.5 to 2%.
(Multiple Exposure of Adjacent Chips)
On the other hand, in transferring a circuit pattern onto a semiconductor substrate using a reflective mask, a plurality of chips of the circuit pattern are formed on a single semiconductor substrate. Between adjacent chips, there may be a region where the outer peripheral portions of the chips overlap with each other. This is caused by high-density arrangement of the chips, which is based on an idea of producing as many chips as possible per wafer to improve productivity. In this case, the overlapped region will be exposed for a plurality of times, four times at maximum (multiple exposure). The outer peripheral portion of each chip of the transferred pattern is also an outer peripheral portion on the mask, which is usually included in the absorbing layer. However, as described above, since the reflectance of EUV light of the absorbing layer is in a range of about 0.5 to 2%, the outer peripheral portion of each chip is problematically multiply exposed. Therefore, it is necessity to provide a region in the outer peripheral portion of each chip on the mask where the effect of shielding EUV light is higher than in a commonly used absorbing layer (hereinafter the region is referred to as a light shielding frame).
In order to improve or even solve such problems, there is proposed a reflective mask in which a groove is formed through the absorbing layer and the multi-layer reflective layer of a reflective mask to thereby lower the reflectance of the multi-layer reflective layer and to provide a light shielding frame having high light shielding properties against the wavelength of an exposure light source (e.g. see JP-A-2009-212220).
However, the EUV light source, which has a peak of its radiation spectrum at a wavelength of 13.5 nm, is known to also radiate light ranging from vacuum ultraviolet outside a waveband of 13.5 nm, which is called out-of-band light, to the near infrared-range light at a wavelength of 140 to 400 nm. In the light shielding frame proposed in JP-A-2009-212220, the out-of-band light is transmitted, as shown in FIG. 12, through the substrate, and reflected off a rear-face conductive film made such as of chromium nitride (CrN) and formed on the EUV mask on a side opposite to a pattern side. Then, the out-of-band light is again transmitted through the substrate for radiation toward a semiconductor substrate to problematically expose the resist coated on the semiconductor substrate.